1. Field of the Invention
This invention relates to a method and apparatus for separation of CO2 and H2S from fluid mixtures containing the CO2 and H2S, such as synthesis gas produced by the gasification of carbonaceous materials and product gases derived from fuel reforming processes. More particularly, this invention relates to CO2— and H2S-selective membranes for high-temperature CO2 and/or H2S separation applications.
2. Description of Related Art
Considering the importance of fossil fuels to the U.S. economy and the impact of anthropogenic CO2 emissions on global warming, developing an effective approach to carbon sequestration, which requires capturing and securely storing CO2 emitted from the combustion of fossil fuels is a matter of great urgency. The overall carbon sequestration scheme comprises two basic elements—(1) separation and capture, including compression, of CO2 from power plants and other emitters and (2) transportation and disposal of the captured CO2 in suitable geological formations or oceans. It is estimated that the costs of CO2 capture and storage would be about $40-$60 per ton of CO2 emissions avoided. Up to 75% of these costs may be associated with the capture and sequestration of CO2 from combustion product gases (i.e. flue gases).
Current options under consideration for separation and capture of CO2 include scrubbing with suitable solvents (either physical or chemical), regenerable sorbents, membranes, cryogenic separation, and pressure and temperature swing adsorption. Of these technologies, solvent-based scrubbing is at present the most mature CO2 separation technology. However, it is not considered to be cost-effective and it is not suitable for use in large-scale power plants. The other technologies are not yet mature or cannot be applied economically at the scale required for power plants.
Various types of CO2-selective membranes are under development, particularly for separating CO2 from fuel/flue gas, with the ultimate objective being sequestration. A major portion of the current work is focused on microporous (pore size less than 2 nm) inorganic membranes based on alumina, zirconia or zeolite membranes supported on porous materials. However, because the separation is based on differences in the physical size, diffusivity and chemical properties of the molecules, separation factors (defined as the ratio of the permeation rate of CO2 to that of the other molecules) are usually low and other molecules, for example hydrogen, permeate along with the CO2. Perovskite oxide-type membranes, such as BaTiO3, have also been studied for CO2 separation because of their excellent stability at high temperatures. However, lower values of CO2/N2 separation factors (1.1-1.2) cast doubt on the potential of the ceramic membranes.
To increase the CO2 separation factor, dense membranes based on ceramic materials have been studied in recent years. In one recent project sponsored by the U.S. Department of Energy, hydrotalcite compounds (HTCs) based on Mg—Al—O oxides (for example, Mg0.16Al0.24(OH)2(CO3)0.120.43H2O) prepared by sol-gel and precipitation methods have been studied as potential CO2 removal membranes in the medium temperature range of about 200-300° C. These mixed-oxide ceramic membranes take advantage of the chemical interaction between acidic CO2 and basic oxides in the HTCs. Activation energy calculations suggest the activated diffusion of CO2 through the intercrystalline region of the HTC. However, the chemical interaction may make the reaction irreversible and would result in lower permeation rates. Dense and dual-phase membranes based on K2CO3-doped Li2ZrO3 have also been studied for high-temperature (500° C.) CO2 separation. Based on the carbonate ion conductivity data, CO2 permeance of about 1×10−7 mol/m2·s·Pa has been calculated at about 500-600° C.